The present invention relates generally to a device for transferring semiconductor wafers. More specifically, the invention relates to a semiconductor wafer transferring device for transferring a plurality of semiconductor wafers between a wafer cassette and a wafer boat or the like, simultaneously.
As is well known, semiconductor wafers for use in IC manufacture, LSI and so forth are held in wafer cassettes before being treated by diffusion or CVD processes or the like. Wafer cassettes are generally designed to hold a large number of semiconductor wafers. For example, a typical wafer cassette can hold 25 semiconductor wafers. Such wafer cassettes generally comprise a pair of holder walls lying vertically and parallel to each other. The lower corners of the holder walls are connected to each other by means of connection strips so as to be held at a fixed spacing. A plurality of wafer retention grooves are formed on the opposing surfaces of the holder walls. Each wafer retention groove extends essentially vertically from the top of the associated holder wall to the vertical center of the holder wall. Semiconductor wafers are vertically held within the wafer cassette, wherein one wafer is disposed in each of the wafer retention grooves of the holder walls.
For diffusion, CVD treatment and so forth, the semiconductor wafers held in the wafer cassette must be transferred to a wafer boat which comprises a heat-resistant material, such as quartz, for placement in a diffusion furnace or the like. After treatment, the semiconductor wafers are removed from the furnace by way of the wafer boat. Thereafter, the treated semiconductor wafers are transferred from the wafer boat back to the wafer cassette.
The wafer boat generally comprises a pair of retention bars lying parallel to each other. The retention bars are connected to each other by means of connecting pieces. The retention bars have a plurality of opposing, regularly spaced wafer retention grooves which retain the semiconductor wafers by their edges.
In order to transfer a plurality of semiconductor wafers between the wafer cassette and the wafer boat, a typical conventional wafer transferring device comprises a pair of opposing retention plates with a plurality of curved grooves. The retention plates of the wafer transferring device can be moved toward and away from each other. At their closest position, the retention plates can grasp the semiconductor wafers therebetween, with the edges of the wafers fitting into the grooves. On the other hand, when the retention walls are shifted away from each other, they release the semiconductor wafers to either the wafer cassette or the wafer boat.
Practical transferral of wafers utilizing the conventional wafer transferring device involves pushing the semiconductor wafers held either by the wafer cassette or the wafer boat upwardly by means of a pusher so that the retention plates can grasp all of the semiconductor wafers simultaneously. Then, the wafer transferring device carries the wafers to the other of the wafer cassette and the wafer boat. Once positioned immediately above the other of the wafer cassette and the wafer boat, the retention walls are separated to release the wafers onto the boat or cassette.
In the case where the semiconductor wafers are to be transferred from the wafer cassette to the wafer boat, the impact due to dropping the wafers onto the wafer boat may be sufficiently hard to chip either the wafer boat and/or the edges of the wafers per se.
In addition, as is well known, so as to minimize the area of contact between the grooves of the retention bars of the wafer boat while retaining the semiconductor wafers in an essentially vertical position, the width of the grooves of the retention bars is selected to leave very little clearance with the surfaces of the semiconductor wafers. Thus, the semiconductor wafers tend to stick to the grooves. As a result, the upward force exerted on the semiconductor wafers held by the retention plates of the wafer transferring device to lift the wafers off the boat may lift the wafer boat as well.